Fuel systems include a vapor canister that is configured for capturing vapor that is emitted through the fuel system. If fluid enters the vapor canister, the vapor canister will not function properly and replacement is typically required. To prevent fluid from entering the vapor canister, a mechanical valve assembly is provided in the system. The mechanical valve assembly includes a moving valve portion that moves to open under pressure in the fuel system to allow vapor to escape while preventing fluid from flowing into the canister.